diff --git a/src/borg/crypto/key.py b/src/borg/crypto/key.py
index 03ed4a3fc..cd9a8b60e 100644
--- a/src/borg/crypto/key.py
+++ b/src/borg/crypto/key.py
@@ -358,7 +358,7 @@ class AESKeyBase(KeyBase):
 
     def encrypt(self, chunk):
         data = self.compressor.compress(chunk)
-        self.nonce_manager.ensure_reservation(num_aes_blocks(len(data)))
+        self.nonce_manager.ensure_reservation(self.cipher.block_count(len(data)))
         return self.cipher.encrypt(data, header=self.TYPE_STR, aad_offset=1)
 
     def decrypt(self, id, data, decompress=True):
diff --git a/src/borg/testsuite/key.py b/src/borg/testsuite/key.py
index 0779fb3ed..27b42fc80 100644
--- a/src/borg/testsuite/key.py
+++ b/src/borg/testsuite/key.py
@@ -13,7 +13,7 @@ from ..crypto.key import PlaintextKey, PassphraseKey, AuthenticatedKey, RepoKey,
 from ..crypto.key import ID_HMAC_SHA_256, ID_BLAKE2b_256
 from ..crypto.key import TAMRequiredError, TAMInvalid, TAMUnsupportedSuiteError, UnsupportedManifestError
 from ..crypto.key import identify_key
-from ..crypto.low_level import bytes_to_long, num_aes_blocks
+from ..crypto.low_level import bytes_to_long
 from ..crypto.low_level import IntegrityError as IntegrityErrorBase
 from ..helpers import IntegrityError
 from ..helpers import Location
@@ -126,7 +126,7 @@ class TestKey:
         assert key.extract_nonce(manifest2) == 1
         iv = key.extract_nonce(manifest)
         key2 = KeyfileKey.detect(self.MockRepository(), manifest)
-        assert bytes_to_long(key2.cipher.next_iv(), 8) >= iv + num_aes_blocks(len(manifest) - KeyfileKey.PAYLOAD_OVERHEAD)
+        assert bytes_to_long(key2.cipher.next_iv(), 8) >= iv + key2.cipher.block_count(len(manifest) - KeyfileKey.PAYLOAD_OVERHEAD)
         # Key data sanity check
         assert len({key2.id_key, key2.enc_key, key2.enc_hmac_key}) == 3
         assert key2.chunk_seed != 0
@@ -199,7 +199,7 @@ class TestKey:
         assert key.extract_nonce(manifest2) == 1
         iv = key.extract_nonce(manifest)
         key2 = PassphraseKey.detect(self.MockRepository(), manifest)
-        assert bytes_to_long(key2.cipher.next_iv(), 8) == iv + num_aes_blocks(len(manifest) - PassphraseKey.PAYLOAD_OVERHEAD)
+        assert bytes_to_long(key2.cipher.next_iv(), 8) == iv + key2.cipher.block_count(len(manifest) - PassphraseKey.PAYLOAD_OVERHEAD)
         assert key.id_key == key2.id_key
         assert key.enc_hmac_key == key2.enc_hmac_key
         assert key.enc_key == key2.enc_key